The senators wrote that they support reserving a portion of available licenses for carriers with limited nationwide low-band holdings "in order to promote competition, increase auction revenues, and protect against highly concentrated spectrum holdings."

The FCC is planning to set aside up to 30 MHz in each market for smaller carriers to bid on once bidding for those markets hits a set threshold. After the threshold is hit, carriers holding at least one-third of the low-band spectrum in that market wouldn’t be allowed to bid.

Both AT&T and Verizon have been outspoken about the FCC's plans to impose aggregation limits, as the nation's two largest carriers would experience the most impact from the limits. However, in some markets, smaller regional carriers like U.S. Cellular would be restricted.

Wheeler said the rules would be needed in order to ensure no one carrier carrier can monopolize any given market.

So upset was AT&T at one point that it suggested it might sit out the 600MHz auction, saying the rules could restrict its ability to bid in markets covering more than 70 percent of the population.

In a statement, the Competitive Carriers Association (CCA), which also supports aggregation limits, thanked the senators for their support.

"Reserving a portion of spectrum for non-dominant carriers once all statutory revenue targets are met is not only good policy, but it is also common sense," wrote Steve Berry, president and CEO of CCA. "This is guaranteed to increase auction participation, and the more participants at auction, the higher the revenues for the US Treasury and greater the benefits for public safety.”

The senators wrote that they support reserving a portion of available licenses for carriers with limited nationwide low-band holdings "in order to promote competition, increase auction revenues, and protect against highly concentrated spectrum holdings."